A database is somewhat analogous to a card catalog of a library. The card catalog cross-indexes books in several ways.
For example, an "author" card exists. This card is located in the catalog at the alphabetical position of the author's surname.
In addition, a "title" card exists, located in the alphabetical position of the book's title. Also, a "subject" card generally exists, which describes the subject of the book, and which is located in the alphabetical position of the subject, such as "pressed flower collecting," for example. Of course, other cards can exist for a given book (for example, books having multiple authors generally have an author card for each).
In database terms, these cards represent "fields" within the database. For example, in a library, to find all of the books written by Ernest Hemingway, one looks up "Hemingway" in the card catalog. All of the library's books written by any Hemingway are identified by respective author cards. In a computer database, a similar search could be done, upon the "author" field.
Similar searches can be done on the title fields, as well as on the subject fields.
Computer databases, in general, have more fields than does the card catalog. For example, if the card catalog were contained within a computer database, there would probably exist a "date" field. To search using the "date" field, one specifies a date, or range of dates, and the database responds by listing all books having a copyright which complies with the specified date, or range.
Presently existing databases have limitations. For example, many databases require that a user undergo significant training in order to learn how to use the data base. Some databases use arcane commands which the user must memorize.
Some databases contain only that information which is owned by the database owner. Returning to the library analogy, if a public library were to computerize its card catalog, by entering all of the information contained within the card catalog into the database program, the database program would, of course, only contain the card catalog information. A user could not, for example, gain access to the card catalog of a second library, in order to locate a book not owned by the first library. The user must go to the second library.